A thread-break sensor of the type here envisaged is known, for example, from German utility model 79 12 423 dated Apr. 28, 1979. This device comprises a swingable member with two symmetrically located pins disposed just downstream of a thread junction for guiding engagement with the yarn in a normal metastable position of that member in which it is held as long as the two threads, bracketed by its pins, are under substantially equal tension. In the event of a difference in tension exceeding a certain threshold, as will be the case upon the occurrence of a thread rupture, the guide member is laterally deflected into an unstable position from which it gravitates into an inverted, stable position by swinging through approximately 180.degree. about a substantially horizontal axis. This causes an inversion of the relative position of the yarn-bracketing pins which thereby become entangled with the remaining, intact thread to inhibit its advance and to cause its break. Such a guide member has also been described in two commonly owned copending U.S. applications filed by me jointly with Herman Guttler, namely Ser. No. 443,561 of Nov. 22, 1982 and Ser. No. 465,922 of Feb. 14, 1983.
It is sometimes desirable to vary the response threshold of such a thread-break sensor in accordance with the type of thread to be used in a given spinning or twisting station equipped therewith. Thus, the threshold of instability ought to be lower with thinner threads under relatively little tension (and, therefore, a rather small stress difference in the event of a break) but should be higher when the tension of each thread may be subject to appreciable variations without a rupture. That threshold is generally determined by the width of a supporting surface of a carrier on which the guide member rests in a centered position and from which it is deflected by a difference between the forces acting upon its pins. Thus, the response threshold of the sensor may be altered by replacing the carrier, e.g. a bolt with a flattened head, by a differently dimensioned carrier designed to support the swingable member. Such a substitution, however, requires the storage of a variety of carriers whose interchange entails considerable down time in the operation of the machine involved.